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Build Your Own Digital Setting Circles

Use the information on these web pages at your own risk. While I have tried to make the instructions as complete and error-free as possible, I will assume no risk for and will not be held liable for damages of any sort which result from your attempt to use this information. Your use of this information constitutes acceptance of this policy. There is always the risk when constructing electronic circuits that a mistake will result in damage to one or more components. Since this project also involves connecting the circuit to a PC, there is the added risk of damage to the PC if the circuit is built incorrectly. If you have questions regarding this project, please use the comment form below to contact me before you proceed.

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What are Digital Setting Circles?

You probably already know that setting circles are markings on a telescope’s mount that help you point the telescope toward a particular object. Most telescope setting circles seem to be too small or poorly-marked to be useful, though. Digital setting circles also help you point the telescope, and they are much more accurate because rotary encoders attached to the telescope’s axes are used to measure the movement of the telescope, and a computer is used to monitor that movement and continuously report the direction that the telescope is pointing to the user. An electronic interface sits between the rotary encoders and the computer, monitoring the rotary encoders for movement and reporting that movement to the computer via the serial port.

Many commercial telescopes come equipped with digital setting circles, but commercial digital setting circles are expensive to add to telescopes that don’t already have them. I designed my own digital setting circles because I didn’t want to pay the steep price for a commercial system, and also because I thought it’d be a fun project.

My Digital Setting Circles System

My digital setting circles system requires four things:

A telescope on a mount. The type of mount (GEM, Dob, etc.) doesn’t matter, as long as you can find a way to connect a rotary encoder to each axis of the mount.

Two rotary encoders, one attached to each axis of the telescope mount in such a way that whenever the telescope moves (including by the clock drive, if any), the rotary encoders turn.

A decoder circuit that monitors the rotation of the encoders and communicates the encoder angles to a computer. This is the part of the system that I’ve designed and is described here.

A computer (PC, Palm, or PocketPC) that is running software that can get the encoder angles from the decoder circuit and compute the celestial coordinates at which the telescope is pointing.

You probably already have the telescope and mount, or you wouldn’t be interested in this project! The rotary encoders can be purchased commercially, but they aren’t cheap–expect to pay around $50 apiece for new ones. The computer can be an old (or new) PC notebook, but Palm and PocketPC handhelds are becoming increasingly popular for use with this system. There’s a fair amount of free or inexpensive software for use with digital setting circles.

That leaves the decoder circuit, and that’s the part of the system that I’ve designed and that you can build for yourself. The remainder of this web site is devoted to describing the decoder circuit and explaining how to build it. The links given above in the Navigation bar discuss the various aspects of this project, and I encourage you to read all of that information before you embark on building a system for yourself.

This is a project that is not terribly difficult to build. A PC board and a preprogrammed microcontroller chip are commercially available, and with those it’s simply a matter of putting the electronic components in the right places on the board and soldering them in place, and constructing a few cables. I figure a couple hundred of these decoder circuits have been built over the past several years. By far the biggest source of trouble for those who build it is in soldering the components on the board. If you’ve never soldered before, try to find someone who can give you a lesson, or at least try to do a few practice solder joints before you start building this circuit. Soldering isn’t difficult once you’ve seen it done and know what a good solder joint looks like.

Rich, the answer to both questions is “no.” ServoCat only supports Argo Navis and Sky Commander, and my interface won’t emulate either of those. And I don’t publish the source code for my driver software.

You mentioned that you were uncertain of the function of the capacitors for the MAX232.

These capacitors are used to make voltage inverters and doublers to provide the necessary voltages for RS-232 transmission while only requiring +5VDC as a power source.

By charging each of the four capacitors to +5V and then connecting a pair of the capacitors in series you get 10V across the pair. By connecting each of the pairs correctly you obtain two new power supply rails internal to the MAX232 of +10V and -10V.

Dave,
I purchased a kit made by Meade back in the late 80’s. This kit is lacking the pcb’s to decode the encoders, but every thng is brad new. Do you think these encoders will work with your decoder, thay were made by HP and have 4 prongs on the bottem, I have all the wireing and these are powered by two AA batteries. I really want to make this work.
Any ideas?
Thank You Mike

The encoders will probably work. What’s important is that they use 5 volts, and that you can tell how to connect them to the circuit. Try looking on the web for a datasheet for them, if they’re labeled with any part number or something. Or maybe Meade can tell you something about them.

Hi Dave!
Thank you sooo much for all of your efforts.
I was wondering if you have any plans to provide a version of your design to communicate directly through Bluetooth so that a Bluetooth to serial adapter is not necessary.
I have an iPod Touch and I would like to use StarMap Pro on my iPod Touch with your encoder interface board and apparently there are no “made for iPod” Bluetooth to serial adapters.
I could create a new circuit board layout.
Thanks!
Craig

Hi Dave,
Well it is all done and tested works great only trouble is the LED do’nt light up. But no biggie. Oh by the way I found a hyperterminal program that works fine for Vista systems it is called Com Test found at http://www.bb-elec.com/tech_articales/faq_cerrent_loop_loopback.asp To test slect com port then type Q works great and it s free. I also used a USB to serial adapter from best buy this is a good adapter and comes with software download to make it work. I’ll keep you informed if I find out more.
Thank You Mike

Mike, thanks for the info, and glad the system is working for you. It’s possible that the LED doesn’t work because you installed it backwards. Or possibly it just isn’t very bright and you need to check it in a dark room (I intentionally specified a 10K resistor so that the LED was not terribly bright, and so the current consumed would be less). Thanks for the link to the terminal software, too.

I have a Meade 2120 10″ SC with an equatorial wedge mount and the standard RA clock drive. I have read through most of your info on the setting circle project and I am very interested. One of my questions to you is after looking over very carefully the RA and DEC mounts as to how I would go about mounting encoders. Would the “E5 Optical Kit Encoder”, or the “E6 Optical Kit Encoder work as well in place of the shaft style encoders that you recommend?

Thank you… Yes they are. The RA clock drive has a 2 inch round plastic cover set into a shallow machined counter bore centered around the top bearing of the drive and was held in place by 2 10-32 SHCS’s. That shaft has a tapped hole in the middle of it and is about an 1/8 inch below the top of the bearing. This makes for a perfect locator for a stepped diameter post to be mounted to. I can make a round spacer plate with a hole through it to fit the counter bore on which to mount the encoder, and the encoder wheel would fit onto the stepped diameter post. This will make for a very neat and clean mounting. The DEC axis pinion on the fork is a similar situation.

Dave these capacitive encoders are used by EZDSC with their setting circles and after reading about them I don’t see any reason why they won’t work with yours as well. The part number at Digikey is 102-1307-ND. Do you know of any reason why they won’t work? They are much cheaper than the US Digital variety.

I can’t say positively that the Digikey encoders are working correctly because I’m still having an unresolved problem. I think it is related to my bluetooth but until I resolve it I can’t recommend the encoders. I get very crazy readings sometimes.

Just wanted to let you know that your circuit works great with Sky Safari and their SkyWire interface cable. I suspect it would also work with their wireless interface, but I didn’t want to spend the extra money to go wirelss. It’s nice to use this on an iPAD since I don’t have to go back and forth between the eyepiece and laptop when I am observing. The only downside to using Sky Safari is that the alignment is a “one point” alignment—you just point the scope at an object, select it in Sky Safari, and hit the “sync” button…so in my case since I am using an old Meade 2120 with an equatorial mount, you need to have your polar alignment pretty close to minimize any drift.–alternatively, you can do a new “sync” as you move to different parts of the sky.

I wanted to thank you for this wonderful website. I built your “Ek Box” five years ago and had it stowed away for 3 years. I took it out a couple of weeks ago – plugged it and it works. I got the wireless SkyFi i.e. the Orion Starseek Wifi module and set it up with your digital setting circles. It works great and is wonderful. My setup includes a Losmandy G11 mount with the DSCH encoders, your DSC and the Orion WiFi module. I used ECU before – that was dung my”PC” days. Having switched over to Mac based systems , it was essential to make the change. The only difference I notice with the Orion Starseek is that there are no arrows to point the direction you have to move the scope – since I am operating it in a “push to” version. I think this has nothing to do with the DSC or the WiFi module – it is the way the software is programmed. I wish they included that feature where the arrows would point the direction you have to move the telescope. Now , I would have to zoom out and identify the relation between the current scope position and the object of interest, before manually slewing the scope. The system works well with my iPOD and my MacBook pro Laptop – both in wireless mode. The Wifi Module comes with the appropriate cables that hooks up to the DB9 port. Also – the new Version of Skysafari has a two point alignment. So everything works fine!

Apparently, some port drivers don’t write their port names into the registry correctly. I’m going to see if I can compensate for this in my software. In the mean time, you should be able to work around this problem like this:

Open up your astro software and access the telescope settings. In the setup dialog for my driver, go ahead and select one of the scrambled entries, and fill out the rest of the dialog correctly and save your settings. Then close your astro software and launch the ASCOM Profile Explorer (under the Tools menu under ASCOM 6.0 on the start menu). Navigate to “ASCOM.EncoderDSC.Telescope” in the “Telescope Drivers” section. On the right-hand side of that screen you’ll see the settings that you just saved. You can manually edit the Port entry to correct it. Once you’ve done that, you can launch your astro software again and the driver should use the correct port, as long as you don’t use the setup dialog for my driver to change any settings. If you have to re-open the setup dialog for my driver, you’ll have to go back and manually edit the Port value again.

I currently have a Sky Commander hooked to my GEM mount. If I leave the unit on for more than 17 hours it will loose its alignment. Is there any time limit for keeping an alignment with your system if I leave it on and connected to a PC?

Bradley, it looks like it would work (I think you’d want the -F model), but I’d have to take a closer look and try one out to know for sure. It’s worth pointing out that this device costs about twice what it would cost to pick up a USB-serial converter. Also, I think using this would make it harder to use bluetooth if you ever decided to go that way. You can easily purchase a bluetooth-serial adapter to plug right into the serial connector on the board, but you couldn’t do the same thing with a USB-bluetooth converter on the board (that converter will only work with a PC). This is the reason I’ve stuck with the serial interface on my board–it’s much easier to adapt to either USB or bluetooth. Once you modify the board by swapping out the serial for something else, you’re pretty-much stuck with whatever you swapped it for.

The DSC board only uses TX, RX, and GND connections. It’s not expecting any handshaking at all. The BT unit you’re trying to use says that it uses TTL levels, so you should take the MAX232 out of the circuit and connect directly to the PIC chip.

Thanks, the serial cable has not been easy to find. I sent an email to Craig to see how I can get my hands on the necessary components to build his system. Will the DSC program for the Palm interface with the DSC using Bluetooth?

Hi Dave! My name is Göran Kajler and I’m living in Gothenburgh of Sweden. I have bought and built your interface, ver3 and connected a pair of Lumicon Sky Vector decoders. I’m doing a bench test with them connected to your program ver 2.2. I have put tooth pickles to axis of the decoders to see approx to where they are pointing. I get confused to see that when moving one decoder, the reading from the other one is also changing. I could accept this if I would use a Dobsonian but not with an Ekvatorial. When starting up with setting properties , I guess you choose between these two systems by turning the R/A decoder in either direction. However there will not be any difference in the performance. Am I thinking right or wrong ?

Hi, I made the arduino connected to xp laptop version of your DSC last year. It all worked fine on the test bench – I found a way to put some plastic gears on the telescope axis to drive the ex-photocopier magnetic encoders I am using. I can see someone has asked before about not having to do a re-alignment everytime on a permanent equatorial mount – but wondered if there was a workaround or someone else has suggested a fix, because my slewing speed is so slow. Maybe something has turned up in the last year? Many thanks for a design and software that worked first time!

Hi Dave:
I have a newbie question for you. I am renovating a 102 cm reflector here at Stockholms Observatory in Saltsjöbaden. The instrument has Stegman AG615 encoders on both the RA and Dec axis. However the computer and board receiving info from them are missing. Do you think your DSCs would interface with the existing cables and be able to read off the positions?

Judging from what I see in this data sheet I found, it doesn’t look like those encoders would be compatible with my board. Those encoders actually transfer their absolute position to whatever they’re connected to via a serial interface. My board is expecting a completely different type of encoder.

Just started using the setting circles on my EQ5 mount yesterday and realised that they’re way to small to be really useful, hence Googling digital setting circles and finding your site.

What you’ve done in providing this web site and a way hobbyist to access this technology on a budget is fantastic and must be applauded.

Just one suggestion to make the whole project better – remove the necessity to use a Microsoft OS/Microsoft Visual C# 2010 Express as it rather corrupts the whole point of the exercise. Would it be possible for you to rewrite the software using something which is OS independent? That way the Ubuntu user of this world wouldn’t feel excluded.

Pete, thanks for your kind words. My ASCOM driver is written specifically for Windows OS’s because, well, ASCOM itself only has a version for Windows (understandable, since it’s based on Microsoft .Net). I’m not aware of an equivalent interface that supports Linux. My interface box does emulate other similar hardware like BBox and NGC Max, though, so if you can find software that supports those protocols directly that will run under Linux (perhaps using Wine?), they *ought* to work with my interface box. You might even try running my DSCWin software (*not* the ASCOM driver) using Wine.

I would like to purchase the Digital setting circles kit, and use your software.
Which kit version should I buy..the USB DSC or the serial DSC for your program? I plan on running it on windows 7. Thanks in advance.

If you ever plan to use bluetooth to link the board with the PC, then buy the serial version and also buy some bluetooth adapters like I describe here. If you buy the serial version, you’ll also have to buy a USB-serial converter because your PC probably doesn’t have a serial port. Buying the serial version with adapters for USB and bluetooth is the most expensive way to go but also the most versatile.

If you don’t plan to use bluetooth, I’d buy the USB version. There’s no easy way to convert to bluetooth using the USB version, just so you know.

Dave, Thanks for your reply. I went ahead and ordered the USB DSC kit and the FTDI TTL-232R-5V cable. Looks like a pretty neat little setup. I am going to replace the Magellan II hand controller with your kit and a laptop. I tested your software on my windows 8 computer, seems to open up just fine, so it should work on the laptop I will use to connect to my Meade 10″ Starfinder EQ telescope.

Bill, are you trying to use the interface through bluetooth? If so, on your laptop go to Control Panel -> Devices and Printers -> and look for your interface there. Right click on it and select Properties, and then look at the Hardware tab in the window that pops up. The COM port should be shown there. And yes, you’ll need the ASCOM platform if you want to use CDC.

hi dave, first i would like to thank you for your help. I have your serial board built i am using bt232b adaptor to communicate to my laptop (window7). I have two questions, First, do i need to set up my bt232 by connecting it to my laptop? And second if the previous is the case i don’t have hyperterminal on my windows 7 laptop to set it up, what do i do?
thanks in advance.bill

As far as Hyperterminal goes, you can either grab a copy from a Windows XP machine, or you can use another terminal emulator to do the job. I’ve recently started using PuTTY, which is a pretty popular solution. You can get it here:

So basically the advantage of commercially sold units such as Argo narvis and NGC max is that you do not have to make them yourself, and they have no need for a computer?
It seems like those are compelling reasons to just go commercial, no?

and I was also going to add to that that commercial computers have large objects lists. But i suppose that does not matter if you are hooked up to a computer.

The reason I did this project over ten years ago was that those commercial units were expensive. It is true that it’s more convenient to go the commercial route, and many modern telescopes (like the Celestron NexStar 6SE I recently purchased for myself) even come with this functionality built in. But if you’re on a budget, trying to upgrade an older or home-built scope, and/or if you just like building things, this project might still appeal to you.

Hi, Dave – Thanks very much for sharing your knowledge and time! I will be adding your DSCs to my 8″ Dobsonian. I notice that on your “ASCOM Driver” page, you mention that the “Slew” command will display a couple of numbers – a user would then manually move the alt/az until the numbers zero. Conceptually, it would seem that the two numbers could in fact be used to command a couple of servo or micro stepper motors to “drive” the scope to that position. I really want to “track” so I can try some astrophotography – I realize that for an alt/az there will still be field rotation, but maybe a 2nd set of DSCs could be used to interface with a “de-rotation motor” and an auto-focuser. Is that dumb?

I figure one step at a time, so I’ll start with the DSCs so I can “point” to known objects, but I would like to understand the framework for “tracking” later as well.

You could indeed use the two numbers to drive a couple servos or motors given a suitable controller board and interface. It’d be some work to set up, but it’s doable. Mel Bartels used to have plans for such a setup. Take a look at this:

Hello all,
I was building a Dobsonian mount using stepper motors Arduino Mega and RAMPS1.4
(usually used in RepRap 3d printers) .
Modifying the sketch I was able to make it work. The position of the steppers motors is send back as if this were the encoder counter, tested indoor, is working.
If someone is interested I will be more then happy to send files photos and DXF plans of this OPEN SOURCE prototype.
Thanks

Hi Dave,
in older posts someone told about Digikey 102-1307-ND capacitive encoders. Maximum resolution for these encoders is 2048cpr. Do you think they should work or is this resolution too low? I would like to install them on my fork mount ( my telescope is an old celestron C8)

Jim, I have not looked into those encoders, although at first glance it seems they might work. I would encourage you to wire them up in a simple circuit to test them before connecting them to the DSC circuit. On thing you might consider is removing the pullup resistors R2-R5 to disconnect the encoder A and B outputs from the +5V pullup. Not sure, though.